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Pacemakers/ICD/Loop

Invasive Tests
 



Cardiac Pacemakers
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What is a natural pacemaker?

The heart's "natural" pacemaker is called the sinoatrial (SA) node or sinus node. It's a small mass of specialized cells in the top of the heart's right atrium (upper chamber). It makes the electrical impulses that cause your heart to beat.

A chamber of the heart contracts when an electrical impulse moves across it. For the heart to beat properly, the signal must travel down a specific path to reach the ventricles, the heart's lower (pumping) chambers.

The natural pacemaker may be defective, causing the heartbeat to be too fast, too slow or irregular. The heart's electrical pathways also may be blocked.  



• Showing the conduction of the electrical activity travelling from the SA Node through the AV Node down through the Purkinje Fibres into the Heart Muscle.


What's an artificial Pacemaker?


An "artificial pacemaker" is a small, battery-operated device that helps the heart beat in a regular rhythm. Some are permanent (internal) and some are temporary (external). They can replace a defective natural pacemaker or blocked pathway.  

  • A pacemaker uses batteries to send electrical impulses to the heart to help it pump properly. An electrode is placed next to the heart wall and small electrical charges travel through the wire to the heart.


  • Most pacemakers are demand pacemakers. They have a sensing device. It turns the signal off when the heartbeat is above a certain level. It turns the signal back on when the heartbeat is too slow.

 

  • There are many types of pacemakers designed to meet the different needs. Some pacemakers are placed for a slow heartbeat or when the heart ‘pauses’ for a period of seconds. Some pacemakers are designed to help heart failure. Other types can help correct irregular heart rhythms or allow certain medications to be used to help regulate heart rhythms.

 



How does a Pacemaker work?

The pacemaker, or generator, is implanted under the skin, usually just below the collar bone (clavicle). Small leads or wire are passed through a nearby vein (subclavian) into the right side of the heart. The tips of these leads have a device than anchors them into the inside wall of the heart. On the tips of the lead is an electrode.

An electrode is a device that allows the pacemaker to sense the electrical activity of the heart and, if needed, deliver electricity to the heart muscle to make it contract or beat. A battery or generator powers the pacemaker. Like any device that is run on a battery charge, the battery of the pacemaker will need to be changed at some time. The usual lifespan of a pacemaker generator is approximately 7 to 12 years, but this time will vary depending on amount of use. The generator may need to be changed as soon as 2 years or as long as 10 plus years.

The function of the pacemaker and the battery life should be monitored on a regular basis. This can be accomplished by a 6-month or annual interrogation of the pacemaker. Changes can be made in the programming of the pacemaker through a process called interrogation. This is a painless process where a programmer is placed on the skin over a pacemaker generator. Information can be extracted from the pacemaker and changes can be made in the timing cycles or other functions of the pacemaker. This process is done by a Technologist, however, sometimes your Cardiologist may do this test in his rooms if he has the necessary programmer.  


What happens when the battery needs to be changed?


Regular interrogation of the pacemaker will notify the physician or cardiac technologist when a generator change is needed. The pacemaker has the ability to “tell” the programming device that the battery is running low and a new battery is needed in the near future. The physician or cardiac technologist can then make arrangements for a battery or generator change. The procedure entails you being admitted into hospital and the procedure is undertaken in a sterile field – the cath lab. The old pouch is re-opened under local anaesthetic. The lead are tested again by the Technologist and if the lead threshold are within normal limits, the leads are left in place and just a new battery is attached to the same old leads.

However, over time, the lead threshold may change due to several factors like myocardial infarction (heart attack) to the muscle where the lead is attached. This will result in higher threshold thus depleting the new pacemaker battery quickly. This old lead is left in place in the heart and is capped (this lead cannot be removed because over the years the distal tip of the lead is embedded into the heart muscle). A new lead is introduced and tested for its thresholds and once optimal thresholds are achieved, the lead is secured in place and attached to the new pacemaker generator (battery).  


Temporary Pacemaker Insertion

• Inserting Temporary Pacemakers


Indications for temporary transvenous ventricular pacing include:

  • Acute MI – symptomatic bradycardia, bilateral bundle branch block, new or unknown duration bifascicular block with first degree AV block.

 

  • Mobitz type II second degree AV block.

 

  • Right bundle branch block with left anterior hemiblock or left posterior hemiblock (new or unknown duration).

 

  • RBBB with first degree AV block.

 

  • LVBB (new or unknown duration).

 

  • Recurrent sinus pauses >3 secs, unresponsive to atropine.

 

  • Bifascicular block of unknown duration.

 

  • RBBB (new or unknown duration).

 

  • Bradycardia not associated with MI - asystole, second or third degree AV block with haemodynamic disturbance or syncope at rest, ventricular tachyarrhythmia, secondary bradycardia.

 

  • Prior to insertion of a Permanent Pacemaker – used as a back-up when testing the permanent lead thresholds.

 

Technical Information

The first pacemakers were developed in the early 1950’s and consisted of the three basic parts that make up today’s pacemakers; pulse generator, wires; and electrodes. Today there are four general classifications of pacemakers; transcutaneous, transthoracic, transesophageal, and transvenous. The two types that are most commonly encountered in transport are the transcutaneous and the transvenous.

Transesophageal
 
Transesophageal pacemakers are inserted via the mouth and the electrode sits in the esophagus. The esophagus lies directly behind the heart and has relatively thin tissue whose moisture is beneficial for signal transmission. This mode of pacing is reliable and easily established emergently however maintaining proper position poses problems and this becomes more of a problem with patients involved in the back of bumpy ambulances!

Transthoracic  
 
Transthoracic pacing requires pacemaker electrodes be inserted through the chest wall and onto the outer surface of the heart – the epicardium. These pacemakers can use the same pulse generators as transvenous pacemakers, but are more problematic. First of all they are usually inserted during cardiac surgery when clear, direct visual access permits proper placement. If the patient is already at a hospital that is capable of cardiothoracic surgery, chances that they will need to be moved, in critical condition, to another facility are low.

The leads for a transthoracic leads may also be inserted in a cardiac arrest victim through incisions in the subxiphoid area, but the success rate is poor. Also, a high skill level is needed to properly insert the leads.

Transcutaneous  
 
Transcutaneous pacemakers deliver the pacing stimulus to the heart through the chest wall via two large adhesive pads. Good contact between the pads and the patient’s skin is important. Electrical burns occur with this type of pacemaker, not because of the amount of energy delivered, but because of the surface area over which the signal is delivered. It is actually the resistance to the flow electricity that generates the heat that causes the burns associated with both transcutaneous pacing.

Because the pacing stimulus travels through chest wall, pectoralis and intercostal muscles, ribs pericardial fluid and fat before finally reaching the heart. This necessitates the use of much higher energy levels to deliver the correct energy to the heart. One side effect of this is chest muscle contraction from the electrical stimulus. Therefore medicating the patient for comfort is high priority.

Transvenous  

 
Transvenous pacemakers are inserted into the venous system through a sheath at the site of the groin (Femoral approach) or the arm ( Subclavian approach). This delivers the stimulus via electrodes that are in direct contact with the heart tissue itself. Since there is direct contact between the electrodes and the heart tissue, the energy levels and the size of the electrodes can be much smaller.

Also patient comfort is improved because chest muscle contraction, although possible is uncommon and usually less intense.  

Pacemaker Classification  
 
Pacemakers can deliver pacing stimuli to either the atria or the ventricles, or both; however in emergency pacing single chamber pacing is generally performed due to difficulty attaining correct lead placement for two leads. Transcutaneous pacing is always single chamber (ventricular) because of the inability to direct the pacing stimulus.  
 
• The First Letter
 
In the three letter naming system jointly developed by NASPE (North American Society of Pacing and Electrophysiology) and the BPG (British Pacing Group) the chamber paced is always listed first and is designated as A for atrium and V for ventricle (D is for dual when pacing both chambers).
 
• The Second Letter
 
The second and third letters are related and tell the user whether the pacemaker can sense what the heart is doing and how the pacemaker responds to this information. The second letter can be one of four; A for atrium, V for ventricle, D for dual and O for none. This letter indicates which chamber the pacemaker is capable of sensing. In single chamber pacemakers this can only be the chamber paced or none, because single chamber pacemakers have only one lead that terminates in the chamber paced. Therefore if the pacemaker is going to be able to sense anything, it must be in the chamber where the lead is located.  
 
• The Third Letter
 
The third letter indicates what the pacemaker does with the sensed information. It can either be I for inhibited, T for triggered, D for dual or both inhibited and triggered, and O for neither. Inhibition occurs when the pacemaker senses the hearts intrinsic beat and stops the pacemaker from discharging. For example, if a ventricular pacemaker senses that an intrinsic beat causes the ventricle to depolarize, then the pacemaker will not deliver a competing stimulus
Triggering occurs when a natural stimulus is not sensed by the pacemaker and triggers the pacemaker to fire. This is usually used to maintain a normal synchronization between atrial contraction and ventricular contraction, and therefore is not used in the single chamber pacemakers we are discussing.
That limits the three letter pacemaker code to AOO, VOO, AAI, or VVI for single chamber pacemakers. These pacemakers are indicated for bradycardic hearts.

AOO and AAI pacemakers are used when the sinus node is dysfunctional such as symptomatic sinus bradycardia or sick sinus syndrome but the AV node conduction is normal.

VOO and VVI pacing is indicated for symptomatic AV node dysfunction such as third degree AV block or any bradycardic rhythm where concern about AV node conduction exists.



 

An atrial pacemaker-  

 
ECG strip showing an atrial pacemaker  

 
A VOO pacemaker. Any underlying intrinsic rythym is ignored by the pacemaker.  

 
A VVI pacemaker. Here the intrinsic rhythym is sensed and the pacemaker does fire.  


 

Demand Mode
 
Pacemakers can be further classified according to the third letter as asynchronous (fixed) or synchronous (demand). Asynchronous (a = not; syn = together; chrono = time) pacemakers are those that are not together in time with the heart because they do not know what the heart is doing. Pacemakers such as AOO and VOO are asynchronous because they do not sense what the heart is doing (second letter O). The mode is known as fixed rate because they will discharge at a fixed interval (depending on the programmed heart rate) regardless of whether the heart needs the help or not.

This is also known as competitive pacing because the pacemaker competes with the patient’s natural rhythm for control of the heart. Competition may be good in sports, but it can be detrimental or even deadly in the heart. AOO pacemakers increase the chance of a patient developing atrial fibrillation because the pacemaker stimulus and the natural stimulus can cause chaos in the atrial tissue. This would decrease the cardiac output from the loss of atrial kick, further compromising cardiac function.

VOO pacemakers also run the risk of inducing chaos – in the ventricles. If the pacemaker should fire at the wrong time, when the ventricles are not ready, the result could be ventricular fibrillation. Should this occur it is terminal without immediate treatment!
AAI and VVI pacemakers significantly decrease the risk of chaos in the pacing chamber by monitoring it for natural activity. Since they are able to monitor the chamber that they wish to pace, they can hold off generating an impulse to eliminate competition. These pacemakers are known as synchronous pacemakers and the mode is referred to as demand pacing; only pacing when the heart demands help because of no natural activity has occurred.
 

Pacemaker Settings

 
Pacemakers are not simply applied to the patient and turned on; we must tell the pacemaker how to pace the heart. The first parameter we can adjust is rate. The goal of pacing the heart is to maximize cardiac output. There are two determinants of cardiac output; stroke volume and heart rate. Since single chamber pacemakers have little impact on stroke volume, we must adjust heart rate to accomplish our goal.
 
Heart Rate

 
Heart rate can be set along a long continuum of values. Some pacemakers allow heart rates as low as 30 bpm and as high as 180 bpm, with allowable atrial rates sometimes up to 800 bpm for short periods of time. The heart rate is usually decided by the physician ordering the pacemaker, but may need to be adjusted, especially in transport when anxiety can increase myocardial oxygen demand.
If the pacemaker is in AOO or VOO mode, then the programmed heart rate is the rate the pacemaker will discharge, whether the heart has a natural rhythm or not, because the pacemaker does not sense the heart in these modes.. With demand pacemakers, AAI and VVI, the programmed heart rate is a minimum allowable heart rate. If the pacemaker does not sense a natural stimulus that will ensure that the heart rate will not drop below the programmed rate it will discharge. Either way, the heart rate should never fall below the programmed heart rate.
 
Pacemaker Output

 
Once the pacemaker has been programmed with the minimum heart rate, we must now tell it how much energy to use when a pacing stimulus is delivered. Many variables can affect the amount of energy necessary to pace the heart from the amount of muscle and fat tissue present for transcutaneous pacing, to the type of electrodes, and scar tissue at the point of contact for transvenous pacing, we simply need to determine the amount of energy that assures the pacemaker stimulates the heart to depolarize.
 
Minimum Pacing Threshold
 
Minimum pacing threshold is the lowest pacemaker energy output that assures that the stimulus depolarizes the target chamber consistently. Because things can change over time the actual pacemaker output is usually set 2 to 3 times higher than this number for a large safety margin.

To determine the minimum pacing threshold we need to assure that the pacemaker will need to discharge. To do this we simply turn the pacemaker heart rate up until it is about 10 bpm faster than the patient's natural heart rate.

We then turn the output down until the ECG shows loss of capture which is evidenced by pacemaker spikes that are not followed by QRS complexes. Once this occurs, we slowly begin increasing the output until the ECG shows full capture of the pacing stimuli. Note the number indicated by the dial, this is the minimum pacing threshold.

Now simply set the output to a value 2 to 3 times greater than this number to ensure capture and reset the pacemaker to the original heart rate.
 
Pacemaker sensitivity

 
The body is full of electrical signals; they drive nerves and muscles as well as the heart. When the pacemaker is trying to sense the heart’s natural rhythms, it needs to know how to differentiate heart signals from all the extraneous non-heart noise that is ever present in and around the body. This is accomplished is by adjusting the sensitivity.

One potentially confusing fact about sensitivity is that the lower the value on the dial, the higher the sensitivity. This counterintuitive fact is easily explained though. Sensitivity is measured in milliamperes (mV) and can range from 0.5 mV (the most sensitive) up to 20 mV (the least sensitive) on most devices. The programmed sensitivity represents how much energy the pacemaker must sense before it recognizes a natural beat. So, lower numbers mean that the pacemaker recognizes lower amounts of energy as natural beats.

If the sensitivity is turned off, then the pacemaker becomes asynchronous, because if the pulse generator can’t sense the heart then it will pace regardless of the heart’s natural activity.
 
Sensing Threshold
 
To set the sensitivity, first it is important to stop the pacemaker from discharging. To accomplish this, first set the minimum heart rate to 10 bpm below the patient's natural rate and turn the output level all the way down. These interventions ensure that, when the pacemaker is not sensing correctly (while we are adjusting the sensitivity) the pacemaker's output will be low enough so that it does not depolarize the heart.

Once the risk of competitive pacing is limited, slowly decrease the sensitivity (turn the dial toward larger numbers) until the pacemaker shows pacing spikes at a set, unchanging interval (the pacemaker is now asynchronously pacing). The sense indicator light will stop flashing. (Capture is not likely because of the low output so if the patient's natural heart rate is too slow the patient may deteriorate so this procedure must be accomplished as quickly as possible).
 
Sensitivity Safety Margin
 
Now increase the sensitivity (turn the dial toward lower numbers) until the sense indicator light begins flashing with every natural heart beat. This is the sensing threshold and indicates the amount of current that reaches the pacemaker electrode when the sensed chamber depolarizes. To build in the necessary safety margin it is important to set the sensitivity to half this value. This ensures that even if the heart’s natural signals have a bit less current they will still be sensed, but most electrical artifact will be ignored. For instance, if the sensing threshold is 10mV, then the sensitivity should be set to 5mV. This will allow for proper sensing even if the intensity of the signal changes slightly from beat to beat or hour to hour. After the sensitivity is properly adjusted reset the heart rate and output levels to the correct values.



 
 
 
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